The present invention relates to an optical system for reading from and/or writing information on an information recording and/or reproducing medium such as an optical disk and, more particularly, to an optical head for projecting a laser beam onto an information recording and/or reproducing medium to read from and write information on it.
In an optical system which reads out from or writes information on an information recording and/or reproducing medium, a laser beam is projected onto the information recording and/or reproducing medium (to be referred to as an optical disk hereinafter) through an optical head. The laser beam reflected from the optical disk is picked up by the optical head. In order to write information on the optical disk and to properly read it out, the focal point of an objective lens of the optical head must be accurately placed on the light reflecting surface of the optical disk. In other words, the beam waist of the laser beam converged by the objective lens must be projected onto the optical disk. Various apparatuses have been developed to detect the defocusing state or out of focus state of the objective lens. However, none of them is satisfactory. For example, an apparatus which utilizes the difference between sizes of focused and defocused beam spots on the optical disk has been proposed wherein different patterns of the focused and defocused beam spots are projected onto a photodetector and are detected by the photodetector to achieve proper focusing. Further, an apparatus for focusing a laser beam, which is provided with a lens system combining a convex lens and a cylindrical lens is disclosed in U.S. Pat. No. 4,079,247 of Bricot et al. In these apparatuses, if minute recesses or projections are formed on the optical disk, a diffraction pattern is formed in the beam spot pattern on the photodetector, resulting in erroneous operation. Especially, in an optical disk which has a tracking guide to increase information recording capacity, a diffraction pattern of the tracking guide is formed in the beam spot pattern on the photodetector when the beam spot is formed on the tracking guide. As a result, the apparatus may be erroneously operated.
In the apparatus described above, the defocusing state of the objective lens is detected by changes in the size of the beam spot pattern on the photodetector or in the shape of the beam spot pattern. Another apparatus has been proposed which detects the defocusing state of the objective lens by a location of a beam spot pattern formed on a photodetector. In this apparatus, a laser beam for detecting the defocusing state of the objective lens is incident on the objective lens parallel to an optical axis of the objective lens and projected onto the optical disk therethrough, in addition to laser beams which are used for readout and writing of information. However, such an optical system becomes complex and the apparatus is manufactured at high cost, because the optical system having at least two optical paths for the laser beams is required. Other apparatuses are disclosed in Japanese Patent Disclosure Nos. 53-28405, 49-31128 and 53-10407, respectively. In these apparatuses, laser beams for reading out and writing information do not travel on an optical axis of an objective lens but rather travel parallel to the axis thereof. However, in these apparatuses the laser beams cannot be sufficiently converged by the objective lens, and a sufficiently small beam spot cannot be formed on the optical disk. Further, since the laser beams pass through the outer peripheral portion of the objective lens and are projected onto the optical disk, the intensity of laser beams projected onto the optical disk is decreased by eclipse.
Further, an apparatus is disclosed in Japanese Patent Disclosure No. 53-118103, in which a prism is arranged on an optical path of laser beams reflected by an optical disk. In this apparatus, the intensity of the laser beams may be attenuated when they pass through the prism, and unwanted diffraction may occur.
Although the aforementioned various drawbacks are peculiar to the optical system of an optical head, the inventor hereof has discovered that the photodetector provided in the system for detecting the defocusing state of the objective lens is also problematic In the conventional optical head, a diffraction pattern appears on the beam spot on the light receiving surface of the photodetector due to projections or recesses on the light reflecting surface and to dirt and defects on the optical disk and in the optical system. Due to the diffraction pattern the photodetector judges the objective lens as being in the defocusing state in spite of the fact that the objective lens is in the just focusing state thereby giving rise to the possibility that a focusing signal will be generated. As a result, it is not possible in the conventional optical head to correctly detect the just focusing state. In the conventional optical head, the light receiving surface is divided into several photosensitive regions, which are generally formed on the surface of a semiconductor substrate. Theoretically, only those light rays which are incident on the photosensitive regions are converted into electrons or holes in the substrate under the photosensitive regions. Practically, however, electrons or holes are produced under an intermediate region between the photosensitive regions by light rays incident on the intermediate region, and thus tend to drift to the regions under the photosensitive regions. Accordingly, the level of a signal from the photosensitive regions fails to accurately correspond to the intensity of the light rays incident on the photosensitive regions, and noise involved in the signal from the photosensitive regions are relatively high. As a result, the objective lens cannot always be maintained in the just focusing state.